1. Field of the Invention
This invention pertains to electrochemistry and, more specifically, to composition control of electrodeposited nickel-cobalt.
2. Description of the Prior Art
Electrodeposited nickel-cobalt (EDNi-Co) alloys are attractive because of their high ambient temperature tensile properties. Codeposition of nickel-cobalt alloys has evolved from very hard, brittle deposits produced in Watts type nickel-cobalt sulfate and chloride electrolytes to ductile deposits produced in nickel-cobalt sulfamate electrolytes. The tensile properties of EDNi-Co are determined by the Ni-Co composition, which was thought to be controlled by the independent electrodeposition variables of current density, agitation rate, and electrolyte composition.
Endicott and Knapp in their paper entitled "Electrodeposits of Nickel-Cobalt Alloys", Plating January 1966, reported on their comprehensive investigation of the electrodeposition variables associated with codeposition of nickel-cobalt in sulfamate electrolytes. They showed that the alloy content was determined by the relative concentration of nickel-cobalt in the electrolyte and the deposit current density. The cobalt content decreased with increasing current density. Agitation is also an important variable controlling the nickel-cobalt ratio of the deposit. Dini, Johnson, and Helms in their report entitled "High Strength Nickel-Cobalt Deposits for Electroforming Applications", Sandia Laboratories, March 1973, observed for a sulfamate nickel-cobalt electrolyte (Ni.sup.++ /Co.sup.++ .about.10) and 25 amps/sq. ft (asf) current density, that cobalt content was 28.5 percent with no agitation, 50 percent with moderate agitation, and 53.5 percent with vigorous agitation.
There have, however, been no investigations performed in which both agitation and current density were independently varied to determine any interrelationship or synergistic effects between current density, agitation, and cobalt content. For example, the influence of current density on deposit composition may be due to increasing concentration polarization with increasing current density and could, therefore, be prevented by adequate electrolyte agitation.
In this specification, EDNi-Co composition will be designated in terms of percent cobalt so that an alloy composition of 45% nickel and 55% cobalt would be written as EDNi-55Co. For cases where a significant composition gradient occurs over a given deposit thickness, the composition will still be designated in terms of percent cobalt. Thus, an alloy specimen which has a composition range of 50 to 55% cobalt would be identified as EDNi-50/55Co.